Due to its inert properties, nitrogen gas has long been a widely used industrial gas. Industrial applications include, for example, packaging of perishable foods, provision of non-explosive atmospheres, reducing atmospheres for soldering and brazing, electronic component manufacture and storage, chemical transferring, sparging and mixing, and tire inflation.
Nitrogen gas, when used for tire inflation, has been known to increase the life of the tire. This is due in part to a reduction in oxidative aging, which is caused by the diffusion of oxygen through the wall of the tire. A tire inflated with nitrogen does not experience oxidative aging to the same level as an air-filled tire and, therefore, the life expectancy of the tire is increased. Additionally, the permeation of oxygen through the wall of the tire reduces the tire inflation pressure. If not corrected, improper inflation can cause uneven tire wear, which also reduces the life of the tire. Tests have shown a significant reduction in tire failure for tires inflated with nitrogen as opposed to air.
Traditionally, nitrogen has been produced by distillation of liquified air, and has been provided to industrial users in high pressure canisters. Typically, these canisters are large and heavy. While nitrogen gas is generally readily available and inexpensive, transportation, storage and rental of nitrogen gas containers can be costly for the industrial user. Also, there is always some danger associated with transportation and handling of high pressure gases. In applications where nitrogen gas must be used in remote locations, the danger, cost and inconvenience of transporting and handling large, heavy, high pressure containers is compounded. An example of such situations is the on site repair of refrigeration systems during which nitrogen gas may be used for purging of refrigeration systems before brazing operations, charging systems for leak checking, and for breaking vacuums in large chillers.
Recently, nitrogen gas has been produced utilizing selectively permeable membranes, such as membranes developed by Dow Chemical Company. To produce nitrogen gas, pressurized air is passed through thin hollow fibers fabricated of the selectively permeable membrane material. Oxygen, water and other gases permeate through the membrane wall of the fibers more rapidly than nitrogen, leaving a stream of substantially pure nitrogen.
This method has been used to produce nitrogen on an industrial scale for subsequent distribution in traditional, high pressure canisters. More recently, membrane nitrogen systems have been available for on site installation by industrial users of nitrogen gas. These systems are generally large and operate upon compressed air available from plant systems on location or separate dedicated compressors. Smaller units have been available for specialized applications, such as blanketing aircraft fuel tanks, and have relied upon local sources of compressed air, such as bleed air from a turbine engine compressor.
U.S. Pat. No. 5,302,189 describes a membrane nitrogen gas generator that is capable of meeting the needs of customers having highly variable flow rates. The nitrogen gas generator includes a storage receiver that is capable of storing excess production in periods of low demand. During periods of peak demand, nitrogen gas is provided from both the membrane generator and the storage receiver.
U.S. Pat. No. 5,388,413 describes a portable nitrogen generator for continuously producing nitrogen at various sites with flow rates controlled by a restrictor assembly adjusted manually to control operation. The nitrogen source uses a membrane for gas separation with air cooling and then air heating to provide proper temperatures to control the membrane temperature for gas separation.
U.S. Pat. No. 5,588,984 describes a system for producing nitrogen gas on a continuous or intermittent basis. The system includes an air compressor, which supplies compressed air to a membrane module. The membrane module separates the nitrogen from the compressed air and discharges oxygen and other gases. The nitrogen gas then flows into a vessel for storage. The system includes a back pressure control valve at both the inlet and outlet of the membrane module. The back pressure valves prevent an over-pressure condition within the system.
Current nitrogen generation systems incorporate an automatic shut off circuit for controlling the flow of compressed air to the nitrogen gas generator. Depending on the demand for nitrogen, the circuit cycles the flow of compressed air through the system. The automatic shut off circuit requires electrical power to operate the circuit and/or provides an electrical signal to control other components within the system.